The present invention concerns a new method of loading solid particles in an enclosure, making it possible to obtain a better distribution of a fluid between the loaded particles.
This process more particularly concerns the loading of fixed-bed reactors of the chemical or electrochemical, petroleum or petrochemical type, with solid particles in divided condition, that can be in the form of beads, grains, cylinders, pellets, rods, or any other form, but of relatively small dimensions. In particular, the particles can be from molecular sieves or grains from solid catalysts, generally extruded, produced either in irregular shape or in mono- or multi-lobe shape, the dimensions of which vary, depending on the case, from a few tens of millimeters to a few centimeters.
It is this application that will be referred to more particularly in the following description, but the method according to the invention applies to the loading of any other type of solid particles in an enclosure.
It is known that the method of loading a reactor that consists of pouring the catalyst by means of a flexible conduit, called xe2x80x9csleevexe2x80x9d or xe2x80x9csockxe2x80x9d (xe2x80x9csock loadingxe2x80x9d), or by using buckets filled with catalyst, produces heterogeneous loading which results in an incomplete chemical reaction and an apparent deactivation of the catalytic mass.
To remedy this disadvantage, and more particularly to increase the homogeneity and density of the catalytic bed, it is also known that the catalyst grains can be loaded by sprinkling in order to obtain the most uniform distribution possible, preferably with a horizontal orientation of the grains, over the entire transverse section of the enclosure to be loaded, as described in U.S. Pat. No. 3,804,273, or U.S. Pat. No. 4,051,019. Other types of equipment have also been proposed for obtaining an overall homogeneous distribution of the whole surface of the bed to be loaded, for example in FR-A-2,319,427.
The Applicant has patented a method and a device for the uniform distribution of a solid in divided form in an enclosure, which are covered by EP-A-007,854 and EP-A-116,246, as well as EP-A-769,462.
All of these loading methods, which are grouped under the name xe2x80x9cdense loadings,xe2x80x9d result in a very significant improvement in the homogeneity of the catalytic mass by decreasing, and even eliminating, the random spaces between the grains that are observed in sock-type loading.
More particularly, the present invention concerns methods of loading by rain-effect, particularly those methods that the Applicant has described in the above-mentioned European patent applications, and more particularly, in EP-A-769,462, known by the name DENSICAT, and which lend themselves to precise adjustments of the distribution of the grains inside an enclosure.
One such device, comprised essentially of a rotating metering device, is placed at the upper part of the reactor beneath the manhole through which the catalyst is poured, for example by a feed hopper. While falling, the grains of catalyst strike rotating deflectors, bouncing laterally off of them to be distributed like raindrops, homogeneously, over the entire transverse section of the reactor. One of the most important parameters to control to obtain a uniform rain-effect is the speed of rotation of the moving element of the meter.
This loading technique, which is completely controlled by the operators, has been successfully used in a large number of chemical reactors, particularly reactors for hydrorefining hydrocarbons, for which reactors it is essential to obtain a dense, homogeneous distribution of the catalyst grains. Indeed, the molecules comprising the liquid load to be treated must have the same probability, at any point of the bed, of encountering the same large number of grains of catalysts, with no spaces randomly distributed between said grains, that is, with no preferential paths that the reaction liquid could take at the expense of an effective and complete chemical reaction.
In pursuing her work on methods of loading catalysts by rain-effect in a reactor operating in bi-phase mode, such as liquid and gas, by co-current or counter-current, the Applicant has confirmed that, in spite of the, preferably horizontal orientation of the grains that is obtained by rain-effect distribution, a new phenomenon appeared, unrelated to the disadvantages previously cited for sock loading and resulting in poor homogeneity of distribution of the reaction liquid within the catalytic mass. In fact, and more particularly in the case of loads of reactors that are quite high, especially when this height is more than twice its diameter, the so-called xe2x80x9cwallxe2x80x9d effects tend to lead the liquid toward the internal wall of the reactor, thus creating new preferential paths that route the liquid at the expense of the catalytic mass as a whole.
This phenomenon, which produces local variations of apparent porosity of the catalytic bed, which porosity becomes maximum in contact with a surface, was determined by H. KOYAMA et al., Oil and Gas Journal, Nov. 13, 1995, p. 82, and appears to a greater or lesser degree regardless of the precise shape and dimensions of the catalyst grains. In the more unfavorable cases, the reaction liquid is caught in the most porous area, that is, near the inner wall of the reactor, and accelerates under the effect of gravity. Moreover, since the concentration of gas is lower in this area of the reactor, there is a strong possibility that the risk of coking along the wall will increase. Hot points also appear in the catalytic mass that is poorly irrigated by the reaction liquid, leading to an instability of the reactor that can result in the deactivation of the catalyst. Additional effects due to poor distribution of the liquid on the upper part of the catalytic bed or, more specifically, the presence of a filter screen at the outlet of the reactor, also disturb the distribution of the liquid in the catalytic mass.
All of these disadvantages, therefore, have a negative impact on the yield of the reaction and on the working life of the catalyst, resulting in consequences that can be significant with respect to the equipment and the quality of the manufactured products, and which therefore become seriously detrimental for the operator. This becomes all the more limiting as the strictness of the specifications increases, and particularly the reduction of sulfur content in gasoils, involves the optimization of the hydrodynamics of the desulfuration reactors.
The present invention seeks to remedy these serious operational disadvantages by temporarily modifying the loading conditions of this reactor, during the process of loading said reactor by rain-effect with the particles of the catalyst, so as to compensate, by appropriate modifications of the profile of the catalytic bed being formed, for the local variations of apparent porosity of the catalytic bed, and therefore to modify the flow conditions of the reaction liquid in the catalytic bed, in order to produce a better distribution of the flow of this liquid within the catalytic mass.
Consequently, an object of the invention is a method of loading an enclosure with solid particles, particularly a chemical reactor with particles of catalyst, for the purpose of improving the flow of a liquid between these particles during the operation of the reactor, in which the particles are distributed homogeneously like rainfall to establish a horizontal bed of particles over the whole section of said enclosure, this method being characterized in that during the loading, the loading conditions are temporarily modified at least one time so that the profile of the surface of the bed of particles forms, at least at the periphery thereof, an angle xcex1 with the horizontal of between 6xc2x0 and 25xc2x0, and preferably between 10xc2x0 and 20xc2x0, and in that the loading conditions are then reinstated in order to obtain a horizontal bed.
After a normal start of the loading process, that is, to form a bed in which the solid particles are preferably distributed horizontally, the operator modifies the loading conditions at least one time in order to obtain a new profile of the surface of the bed of particles, preferably when the height of said bed remaining to be loaded is between 0.75 and 2.5 times the diameter of the reactor, and even more preferably between 1 and 2 times this diameter. In effect, it is by temporarily modifying the loading conditions so that the profile of the surface of the bed of particles makes, at least at the periphery thereof, a negative angle with the horizontal in order to obtain a concave loading profile, that is, a profile that slopes the surface of the catalytic bed being formed, from the wall of the enclosure toward the interior thereof, in order to form a depression in the central part of the surface of the catalytic bed, so that the reaction liquid can be redirected toward the center of the catalytic mass, thus minimizing the effects of preferential flows due to the wall of the reactor.
According to the present invention, the operator can also correct the effects of aspiration due to the filter screen located at the outlet of the reactor, generally along the longitudinal axis of said reactor. Indeed, it is known that the presence of a filter screen at the outlet of a reactor, which filter screen is composed of a plate perforated with a plurality of holes that can be of diameters that are different from each other, prompts the reaction liquid to use the preferential paths, i.e., the shortest paths to the filter screen, at the expense of the surrounding catalytic mass. By giving the surface of the bed of particles a profile making a positive angle with the horizontal, at least at the periphery thereof, that is, a convex type loading profile with the surface of the bed of particles sloped toward the bottom of the enclosure near the wall thereof, so as to form a dome in the central part of the surface of the catalytic bed, the liquid can be redirected toward the grains of catalysts that are outermost with reference to the longitudinal axis of the reactor, thus allowing a more homogeneous circulation of the reaction liquid in the entire mass of the catalytic bed.
Preferably, the modification of the loading conditions in order to obtain a convex profile of the surface of the catalytic bed occurs when the loading height of the bed of particles is less than 0.5 times the diameter of the reactor, and even more preferably less than 0.2 times said diameter.
Irrespective of the shape of the loading profile adopted during the temporary modification of the loading conditions, the settings of the process conditions of the rain-effect distribution device for the catalyst grains on the transverse section of the enclosure are selected so that the angle formed between the axis of the slope of the surface of the bed of particles and the horizontal, whether said angle is positive or negative, is between 6xc2x0 and 25xc2x0, and preferably between 10xc2x0 and 20xc2x0.
A loading profile resulting both from concave and convex loading can also be obtained with specific adjustments of the moving device that provides the rain-effect distribution of the catalyst grains.
Predetermined locations of the surface of the bed of particles being formed, to which a temporary modification of the loading conditions can also be applied, can be moved off center with reference to the longitudinal axis of the reactor or composed of at least a portion of the surface of the bed, such as a ring, or parts of this surface contiguous with the inner face of the wall or walls of the enclosure, where it is desired to form a profile sloped from said walls toward the interior of the enclosure or vice versa.
After these temporary modifications of the loading conditions, the initial settings of the device for the homogenous distribution of catalyst grains by rain-effect are reinstated to obtain a horizontal profile of said catalytic bed.
All of the rain-effect modes of loading implemented in the standard way, that is, for which the grain distribution device allows a homogeneous distribution over the entire transverse section of the reactor or enclosure to be charged, can be used according to the invention.
A particularly preferred rain-effect loading mode uses a rotary distribution device placed beneath a feed system for particles of the enclosure, the speed of rotation of which is an adjustable parameter. Preferably, the device comprises flexible deflectors suspended from a shaft capable of rotation and which are articulated on said shaft so as to rise under the effect of centrifugal force. This technique is perfectly controllable by the operators and lends itself with no difficulty whatever to precise temporary modifications of the loading conditions, at predetermined locations of the height of the catalytic bed in the process of being formed, in order to modify the profile of the surface of said catalytic bed.
With this preferred mode of loading, a convex profile of the catalytic bed being loaded can be obtained by decreasing the speed of rotation of the rotating element of the Densicat device, while a concave profile is obtained by increasing this speed of rotation, with reference to the nominal required speed normally used to obtain a horizontal loading profile.
The modification of the loading conditions for the enclosure, in order to locally vary the profile of the surface of the bed of particles in process of formation, can occur one or more times when the level of the bed of loaded particles in this enclosure reaches a predetermined level in the column.
One form of implementation of the invention will now be described by way of example, with reference to the appended diagrammatic drawings.